In gas phase polymerization, a gaseous stream containing one or more monomers is passed through a fluidized bed under reactive conditions in the presence of a catalyst. A polymer product is withdrawn from the reactor. Fresh monomer is introduced to the reactor to replace the removed polymer product, and unreacted monomer is recycled back to the reactor. Process upsets in an ancillary system upstream and/or downstream of the reactor may require the polymerization to be shutdown or “killed.” The use of multiple catalysts in the same reactor system may also require shutdowns, for example, to transition from one catalyst to another.
In the case of transitioning between catalysts, it is desirable to minimize the complexity and amount of time required for the transition and to minimize the amount of off-grade resin product produced. Many transition procedures are directed toward accomplishing these goals. However, typical kill procedures still often require the reactor to be opened, purged of hydrocarbons, emptied of polymer and catalyst particles, cleaned, and reloaded with the removed bed or a new bed to provide a “seedbed” of polymer. This process is time consuming, expensive, and allows impurities, such as moisture and air, to enter the reactor. Such impurities necessitate another time consuming procedure to remove.
WO 2004/060930 discloses a process for transitioning between a metallocene and a Ziegler-Natta polymerization catalyst system. The process requires discontinuing introduction of a metallocene catalyst, introducing a catalyst deactivating agent to stop the metallocene polymerization reaction, purging the reactor with an inert gas to remove unreacted deactivating agent, and then introducing a Ziegler-Natta catalyst to the reactor. US 2004/0138391 discloses a process for transitioning between catalyst systems that are incompatible with each other in a gas phase fluidized bed reactor containing a fluidized bed of polymer particles. The process requires continually passing monomer gases through the polymerization zone, discontinuing the introduction of a first catalyst system into the reactor, lowering the height of the bed of polymer particles, introducing a second catalyst system into the reactor, and then increasing the height of the bed of polymer particles. WO 2011/103280 provides a method for shutting down and restarting a gas phase polymerization reactor. The method involves introducing a polymerization neutralizer to stop polymerization. The method can also include stopping recovery of a polymer product from the reactor, stopping introduction of a catalyst and reactor feed to the reactor, and adjusting the pressure and superficial velocity of cycle fluid through the reactor from operating to idling levels.
Shutdown or transition procedures in gas phase polymerization reactors are often accompanied with a buildup of catalyst and polymer on the walls of the reactor, which is known as “sheeting.” Another common problem is the buildup of catalyst and polymer on the internal distribution plate, injection nozzle(s), and/or product discharge nozzle(s), which is known as “plugging” or “plate fouling.” Sheeting, fouling, and plugging can force a complete reactor shutdown for cleaning and removal of the polymer chunks, which could take several days. This undermines the efficiency of any process designed to minimize transition time.
It is particularly difficult to control sheeting, fouling, and plugging with metallocene catalysts during reactor shutdowns or transitions because they are known to exhibit unpredictable static tendencies. For instance, EP 0 811 638 describes metallocene catalysts as exhibiting sudden erratic static charge behavior that can appear even after long periods of stable behavior. It has been found that many of the known methods of shutting down or transitioning a reactor from one catalyst to another fail to prevent sheeting and the like with transitions between metallocene catalysts. There is a need for improved methods for transitioning a gas phase polymerization reactor system between metallocene catalysts.